1. Field of the Invention
This invention relates to engine control systems and, in particular, to a method and system for controlling an engine to maintain optimal oxygen storage levels in an emission control device.
2. Discussion of Related Art
Conventional vehicles typically employ an emission control device such as a three-way catalytic converter to control emissions resulting from the combustion process in an internal combustion engine. When a rich air fuel mixture is present during the combustion process, the emission control device oxidizes hydrocarbon (HC) and carbon monoxide (CO) emissions. When a lean air fuel mixture is present during the combustion process, the emission control device takes up oxygen from the exhaust gas to reduce nitrous oxide (NOx) emissions.
The emission control device is designed to store a finite amount of oxygen. If the maximum oxygen storage capacity of the device is attained during a lean transient of the engine a NOx breakthrough will occur. If there is too little oxygen in the device during a rich transient, a HC and CO breakthrough will occur. Accordingly, engine control systems are designed to control the engine directly, and emission control device indirectly, in order to achieve both maximum robustness and minimum emissions. The term “robustness” is used in the sense of a control system that is tolerant (insensitive) to the impact of air-fuel charge formation due to normal variations in variables whose inherent uncertainty affects the operation of the engine (e.g., measured quantities such as air flow, metered quantities such as fuel flow, environmental variables such as ambient humidity and/or air temperature and manufacturing tolerances such as a sensor time constraint). These goals can be achieved by controlling the engine so as to maintain the oxygen storage level in the emission control device at about 30% to about 70% of the maximum oxygen storage capacity. Within this range, high conversion efficiencies for both NOx emissions and HC and CO emissions are achieved.
Conventional engine control systems have attempted to achieve high conversion efficiencies for both NOx emission and HC and CO emissions using a feedback from an oxygen sensor located in the exhaust manifold of the engine upstream of the emission control device. This oxygen sensor, however, can provide erroneous signals regarding the engine air fuel mixture because of non-equilibrium effects in the exhaust gas. As a result, control systems have been developed that use a secondary oxygen sensor downstream of the emission control device to compensate for the error in the primary oxygen sensor. Use of the secondary oxygen sensor, however, presents its own problems. First, there is a relatively long time delay for the exhaust gas to travel from the engine to the secondary oxygen sensor. Second, when the oxygen storage level of the emission control device is at optimal levels for high conversion efficiencies of both NOx emissions and HC and CO emissions, the air fuel ratio determined by the secondary oxygen sensor is nearly stoichiometric regardless of the air fuel ratio detected by the primary oxygen sensor and the exact value of the oxygen storage level in the emission control device. In closed loop control systems, these characteristics can result in limit cycles in the commanded air fuel signals for the engine and, consequently, engine torque. Use of a secondary oxygen sensor, therefore, typically requires the control system to balance between emissions control and drivability.
In order to optimally manage the balance between emissions control and drivability, control systems have been developed that employ models of the emission control device to estimate the actual oxygen storage level in the emission control device. See, e.g., E. P. Brandt, et al. “Dynamic Modeling of a Three-way Catalyst for SI Engine Exhaust Emission Control,” IEEE Transactions on Control System Technology, Vol. 8, No. 5 (2000), the entire disclosure of which is incorporated herein by reference. These conventional control systems, however, have several problems. First, the model of the emission control device typically uses the signal from the primary oxygen sensor. As discussed above, this signal is corrupted by a bias error that reduces the accuracy of the control system. Second, the models fail to account for variations in the observability of the oxygen storage level in the emission control device.
The inventors herein have recognized a need for a method and system for controlling an engine that will minimize and/or eliminate one or more of the above-identified deficiencies.